1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine. More specifically, the present invention relates to an air-fuel ratio feedback control system for an internal combustion engine which enables the center of the air-fuel ratio control to follow a target air-fuel ratio in an improved manner.
2. Description of the Prior Art
Conventionally, there has been proposed an air-fuel ratio control system for an internal combustion engine in which the air-fuel ratio control is performed by comparing an output signal of an oxygen sensor with a reference value representing a stoichiometric air-fuel ratio so as to determine whether a mixture gas is LEAN or RICH. In an air-fuel ratio control system of this type, a feedback correction coefficient rapidly changes when the monitored air-fuel ratio changes between LEAN and RICH. Furthermore, the feedback correction coefficient gradually changes by an integral action so as to maintain the monitored actual air-fuel ratio at the stoichiometric value. However, a problem with this type of air-fuel ratio control system is its poor follow-up characteristic for controlling the air-fuel ratio toward the stoichiometric value. This is particularly significant when a base of the air-fuel ratio control is deviated due to uneven individual characteristics of the fuel injectors so that the above-noted rapid change and integral action based control cannot follow such a deviation quickly.
In order to eliminate this problem, there has been proposed another type of the air-fuel ratio control system as disclosed in a Japanese First (unexamined) Patent Publication No. 1-121541 and U.S. Pat. No. 4,917,067, which is the equivalent of the former. In the system, the air-fuel ratio control is performed using a pre-stored characteristic which defines a substantially linear relation between an output signal of the oxygen sensor and a "for-control" air-fuel ratio. This pre-stored characteristic, the for-control air-fuel ratio, has a smooth correspondence to variations of the oxygen sensor output signal irrespective of whether it is close to or remote to a stoichiometric value. Accordingly, as the value of the oxygen sensor output signal is deviated from the stoichiometric air-fuel ratio, a value of the for-control air-fuel ratio is also deviated from the stoichiometric value. In this prior art system, since the for-control air-fuel ratio is derived from the oxygen sensor output signal using the above-noted pre-stored characteristic and the air-fuel ratio feedback control is performed based on a deviation between the for-control air-fuel ratio and a target air-fuel ratio, the follow-up controllability of the system is improved.
However, though the prior art air-fuel ratio control system improves the follow-up controllability as described above, there is another problem. An unexpected occurrence of shift or unevenness in the level of the oxygen sensor output signal which may be due to the individual characteristics of the employed oxygen sensor or due to inaccurately measuring temperatures or the like, regardless, the control performance of the system inevitably becomes unreliable. This adversely affects the exhaust emission and the follow-up controllability of the system. FIG. 12 shows this unevenness or shift of the oxygen sensor output. As seen in FIG. 12, the oxygen sensor output signal VOX is considered to be stable during a given air-fuel ratio range across the stoichiometric air-fuel ratio. On the other hand, the oxygen sensor output signal VOX is significantly unstable outside the given air-fuel ratio range. This instability causes the above-mentioned problem.
Further, the dynamic characteristic of the oxygen sensor at the time of inversion from RICH to LEAN differs from that at the time of inversion from LEAN to RICH. Generally, a response time of an oxygen sensor is longer to change its output voltage from RICH to LEAN than from LEAN to RICH. As a result, in the prior art system, the center of the air-fuel ratio control tends to be shifted to the LEAN side so that exhaust emission is deteriorated.
When an engine is idling, it is required that the control amplitude is small so as to provide idling stability. That is, engine speed variations should be set small. However, since the prior art system executes the same control when the engine is idling and when the engine is not idling, the control amplitude when the engine is idling becomes large and deteriorates the idling stability. In order to overcome this problem, in the foregoing prior art air-fuel ratio control system using the rapid change and integral action, the feedback correction coefficient is held fixed after the rapid change action to prevent reflection of the integral action onto the feedback correction coefficient so as to suppress the control amplitude. This, however, deteriorates the follow-up controllability of the system.
Further, due to the individual proper characteristic of each engine, the optimum center of the air-fuel ratio control, which can control the exhaust emission into the regulated range, differs for each engine. Accordingly, a particular means is necessary for shifting the control center to the optimum value required for each engine. In the prior art system, however, since no such a means is provided, the engine's individual characteristic cannot be dealt with.
Further, in the air-fuel ratio control, the required control characteristics differ at engine transitional conditions, such as immediate acceleration, a steady engine condition or during normal driving. Specifically, at an engine transitional condition, the target air-fuel ratio is largely deviated from the stoichiometric air-fuel ratio so that a quick follow-up of the control is required. On the other hand, at a steady engine condition, the actual air-fuel ratio should be stably maintained at the stoichiometric value without being adversely affected by the individual characteristics of the oxygen sensor. However, the prior art system performs the same control both at the engine transitional conditions and at engine steady conditions so that the system is unable to provide the air-fuel ratio control which matches the driving conditions of the engine.